Separation of gas mixtures



Dec. 8, 1959 P. M. SCHUFTAN ETAL 2,915,380

SEPARATION OF GAS MIXTURES Filed May 11, 1956 C 55 r uc 3 $0 "5% LUKD555 L. o v tDO I g 62 rL a f E E uL'J U s ,0 2 x 3 g 2 Inventor.

FAl/L MAURICE .SC/{flflA/v joy/Y MF/LL Attorney removed during the coldperiod; if this were not so,

regenerators or exchanger would soon become completely blocked withdeposit.

2,915,880 SEPARATION OF GAS MIXTURES Richmond Hill, and John Neill,assignors to The British a British company Paul Maurice Schuftan,

Wolverhampton, England, Oxygen Company Limited,

Claims priority,

The present invention relates to the separation of gas mixtures byliquefaction and subsequent rectification and more particularly to aseparation process of this type in which the initial gas mixture aftercompression is cooled by heat exchange with one or more of the coldseparation products in regenerators or reversible heat exchangers.

As is well known, regenerators or reversible heat exchangers serve alsoto purify the gas mixture by freezing out from it any readilycondensible impurities. For example, in the case of air separation, thewater and carbon dioxide in the incoming air are condensed and frozenout on the regenerator or exchanger matrix in the warm period and duringthe subsequent cold period are evaporated or 'sublimed into the outgoingseparation products. In this way, separate purification of the air ismade unnecessary.

For satisfactory operation it is necessary that the material depositedduring the warm period he completfklly Owing to the temperaturedifference between the incoming warm gas mixture and the cold separationproducts at any point in the regenerator or exchanger, the deposits willnot be completely removed unless steps are taken to ensure that the heatcapacity of the cold product stream is not less than that of theincoming warm gas stream and that the volume is greater than that of theof the cold product stream main gas stream. This operation is termedbalancing.

It has previously been proposed to effect balancing by introducing intothe separation plant a subsidiary stream of gas which has been cooledandpurified by some means other than the regenerators or reversingexchanger.

This stream hereinafter termed the balancing stream then serves toaugment the quantity of separation products returning through theregenerators or reversible exchanger.

According to the invention, a gas separation process of the typespecified comprises producing the balancing stream by compressing to arelatively high pressure a minor part of the gas mixture with cooling toambient temperature and removal of condensible impurities, iseniltropically expanding (as hereinafter defined) the whole of suchcompressed balancing stream, and subjecting the expanded balancingstream to rectification in conjunction with the major portion of the gasmixture which has been cooled in the regenerators or reversible heatexchangers.

The term isentropic expansion as used herein implies expansion in amachine with the recovery of external work and the production of coldand is to be understood to include the entropy increase inevitably re-United States Patent 2,915,880 Patented Dec. 8, 1959 ice sulting fromthe imperfections known to exist in all suc machines.

As distinct from previous methods, the balancing stream as produced bythe process of the present invention makes it possible to derive a partof the cold requirements for the process in a highly efficient manner asthe whole of it is isentropically expanded. The amount of a gas mixtureto be diverted to produce the balancing stream and the pressure to whichsuch stream is compressed are determined by the magnitude of the coldlosses and by the necessity or otherwise of recovering separationproducts at least partially in the liquid state. In cases where aseparation product is to be recovered in the liquid state, the amount ofgas diverted to the balancing system must be augmented by an amountequivalent to that withdrawn as liquid.

As applied to the separation of air, the balancing stream is produced bycompressing to a relatively high pressure a minor part of the air to beseparated with removal of water vapour and carbon dioxide as condensibleimpurities, isentropically expanding the whole of such compressedbalancing air and subjecting it to rectitfication in conjunction withthe major portion of the air which has been cooled in the regeneratorsor reversible exchangers.

The isentropic expansion of the balancing stream can be effected eitherin a turbine or reciprocating engine.

The balancing stream after expansion but prior to its entry into therectification zone, may serve other purposes in the separation cycle.For example, where a part of the-gas mixture to be separated is expandedinto the rectification zone through a turbine, the balancing stream mayconveniently be used to provide the preheating of the gas stream priorto its passage through the turbine; this step is necessary to preventcondensation in the turbine.

Again, where residual impurities are removed from the gas stream leavingthe regenerators or reversible exchangers by washing with a stream ofliquefied gas followed by filtration of the liquid, the balancing streammay be used to provide liquid for this purpose.

A further advantage of the present invention is that the pressure andvolume flow of the balancing stream can be easily adjusted and by thismeans the ratio of liquid produced to gas produced can be varied, thusincreasing the flexibility of the plant.

By way of example the invention will now be more particularly describedwith reference to the accompanying drawing, which illustratesdiagrammatically the invention as applied to an air separation processto produce a gaseous oxygen and a gaseous nitrogen fraction.

For the sake of clarity and to avoid undue elaboration of thedescription several features for carrying the process into effect, whichare well known per se to those skilled in the art, have been omittedfrom the drawings. It will be appreciated, for example, that thetemperature of the waste nitrogen must be adjusted before entering theregenerators and that a change-over valve mechanism has to be providedfor sets of associated regenerators. Such features are well known andhave been omitted in the interests of simplicity. In the drawingregenerators are shown as alternating heat exchangers and each pair ofassociated regenerators bears the same reference numeral and are markedrespectively (a) or (b), the suflix (a) being applied to the coolingregenerator and (b) to the generator which is in the course of beingcooled.

; 'te'n'ded to include reversible be used to performother duties, suchDirection of flow along-the interconnecting pipe work iseverywhere shownby arrows. 1 By the term equaliser as used herein, is means an apparatusfor bringing into intimate contact vapour and liquid streams, in orderto smooth out fluctuations in the vapour temperature and to scrub thevapour with the 3 liquid thus removing impurities from the vapour.

It is to be understood that the term regeneratoris inheat exchangers;The air entering the plant is compressed in: a eom- 'pressor 14'anddivided into two major streams, one at about 2.6 atma. (hereinaftercalled-low pressure air), the other at about 4.8 atma.

mediate pressure air). V

The low pressure air is cooled inregenerator 1a to a temperature nearits liquefaction point and is scrubbed The liquid air leaving theequaliser is filtered in' filter the rectification column 7 at about thesame'pointl' The intermediate pressure air is cooled in a similar11113111161 in regenerators 2 and 3 and scrubbed with'liquid(hereinafter 1 called interviding said compressed 13 and passed into Yair in an equaliser 8. :The intermediate pressure air va- I :pourleaving the equaliser is condensed ina condenser 9'by indirect heatexchange with boilingloxy gen in the -,column 7 and the resultant liquidsub-cooled in a subficooler 15 before expansion into the top of thecolumn 7, where it serves as liquid reflux for th'e'se'pa'ration Theliquid air leaving the equaliser 8 is passed to the equaliser 4.

A small fraction of theintermediate pressure airfis diverted before theregenerators, compressed to a high c .7 4 minor stream, compressing saidminor stream to a pressure substantially higher than that of said majorstream, cooling said compressed minor stream to ambient temperature, andremoving condensible impurities therefrom, isentropically expanding thewhole of said minor Stream, cooling said major stream by heat exchangewith at least one of the cold separation products, heating at least apart of said cooled major stream by heat exchange with said expandedminor stream, expanding said heated part of said major stream in aturbine, subjecting both the major and the minor stream to rectificationin a rectification zone and-withdrawing from said rectification zone alower boiling point fraction'and a higher boiling point fraction; i I i-2. The method of separating a gas mixture into a lower boiling pointfraction and a=higher boiling point fraction which compriseswompressingthe gas mixture, di-

gas mixture into a major and a minor stream, compressing said minorstream ,to a pressure substantially higher t-han that of said majorstream, cooling said compressed minor stream to ambient temperature, andremoving condensible impurities therefrom, isentropically expanding .thewhole of said minor stream, cooling" said major streamrby, heatexchange-with at least one of the cold separation products, heatingapart of said cooled major stream by'heat exchange WithISaid' expandedminor stream,'whereby said expanded minor stream is cooled and partiallyscondenscd, expand-ing said heated part of said major stream in aturbine, expanding said cooled minor stream,vwashing the remainder ofsaid major stream with the condensedpart of saidrninor stream to iremove residual impurities from said; re-

' mainder of said major stream, filtering said condensed part of saidminor stream to'remove said residual irnpurities, subjecting the whole.of both the major and minor streains to rectification in arectification zone, and

pressure in a compressor 11, cooled to ambient tempera- J ture in anaftercooler 11a,'freed from carbon dioxide in a carbon dioxide removaltower 11b, dried in one 'of a pair of alternating driers 11c, andexpanded in'an ex pansion engine 12. 'The, relatively cold expanded highpressure air from the engine is further cooled'and par: tially liquefiedin enchanger 5'by heat exchange withthe" low pressure air and expandedthrough a valve l5 into the equaliser 8 where it'serves to'scrub theintermediate pressure air from the regenerators.

j The expanded high pressure air may, if desired, also. as heating thewaste nitrogen passing to the regenerators. The air streams entering thecolumn 7 'are separated into the required waste nitrogen and gaseousoxygen products. The waste nitrogen'leaving the column is warmed, in thesub-cooler 10 and heated to substantially atmospherio temperature in theregenerators 1b and 2b. A

' gaseous oxygen fraction is withdrawn'from the column 7 I and similarlyheated in reg enerator 3b. ;Any liquid oxygen produced is extracted fromthe'column sump and passed to storage. r

, With the process as described above, the necessary excess ofseparation products returning back through the regenerators is achievedand the regenerators balanced.

' The quantity of high pressure air is adjusted according withdrawingfromvsaid rectification zone a lower-boiling point fraction and a higherboiling point fraction.

3. The method of separating air into an oxygen fraction and a nitrogenfraction'which comprises-compressing the air, dividing said compressedair into; a major and a minor stream, compressing said minor stream to apressure substantially higher'than' that of said major stream, coolingsaid compressed'minor strearn to arnbient temperature, and removingcarbon dioxide and water therefrom, isentropically expanding the Whole,of

said minor stream, cooling said major stream by heat exchange with atleast one of the cold separation products, heating at least a part ofsaid cooled major stream by heat exchange with said expanded minorstream, expanding said heated part'of said majorstream in a tur- ,bine,subjecting both said major and said minor stream to rectification in 'arectification zone; and withdrawing from said rectification zone anitrogen fraction and an oxygen fraction. g 4. The method of separatingair into an oxygen fvraca tion and a nitrogen fraction which comprisescompress-v ing the air, dividing said compressed air into amajor 7 1 anda minor stream, compressing said minor streamito to the amount of liquidproducts produced. vIn addie p tion, the expanded high pressure airperforms the func- :tions of turbine preheating and-is a source ofliquid for scrubbing the gas streams leaving the regenerators. While theabove described example uses' a single rec-i. tification column, it willbe appreciated that the present invention can beequally applied 'to asystemusing" a conventional double column f j r We claim? 1. The 'methodof separating ,a'gas mixture into a 7 lower boiling point vfraction anda higher bo iling point fraction Whichcomprises compressing the gasmixture, 'dividingsaid compressed gas mixture intovarnajor and a 1 bienttemperature, and removing carbon dioxide Water-therefrom,isentropicallyexpanding'the whole pf said minor stream; cooling saidmajor stream byflheat exchangewith at leastone of the cold separationproda pressure substantiallyhigher than that of said rhajor stream,cooling said compressed minor stream to am :and

, ucts, heating a part of saidcooled major air-stream by heat exchangewith said expanded minor stream, whereby said expandedfminorstream iscooled and-partially cqndensed, expandin'g said heated part'ofsaid'major stream in a turbine, expanding-said cooled minor stre am;washing the remainder of said'major stream with ithe icon densed part'of said minor stream to, remove residual impurities fromthe saidremainder ofsaidmajorstream,

, filtering said condensed part of said minorsstlteam to, 1e,-

move said residual impurities therefrom; subjeetjing ithe 5 whole ofboth the major and minor streams to rectification in a rectificationzone, and withdrawing from said rectification zone a nitrogen fractionand an oxygen fraction.

References Cited in the file of this patent UNITED STATES PATENTS2,355,660 Le Rouge Aug. 15, 1944 6 Rice Jan. 5, 1954 Skaperdas Feb. 9,1954 Karwat Jan. 11, 1955 Wucherer July 12, 1955 FOREIGN PATENTS ItalyAug. 18, 1952

1. THE METHOD OF SEPARATING A GAS MIXTURE INTO A LOWER BOILING POINTFRACTION AND A HIGHER BOILING POINT FRACTION WHICH COMPRISES COMPRESSINGTHE GAS MIXTURE, DIVIDING SAID COMPRESSING GAS MIXTURE INTO A MAJOR ANDA MINOR STREAM, COMPRESSING SAID MINOR STREAM TO A PRESSURESUBSTANTIALLY HIGHER THAN THAT OF SAID MAJOR STREAM, COOLING SAIDCOMPRESSED MINOR STREAM TO AMBIENT TEMPERATURED, AND REMOVINGCONDENSIBLE IMPURITIES THEREFROM, ISENTROPICALLY EXPANDING THE WHOLE OFSAID MINOR STREAM, COOLING SAID MAJOR STREAM BY HEAT EXCHANGE WITH ATLEAST ONE OF THE COLD SEPARATION PRODUCTS, HEATING AT LEAST A PART OFSAID COOLED MAJOR STREAM BY HEAT EXCHANGE WITH SAID EXPANDED MINORSTREAM, EXPANDING SAID HEATED PART OF SAID MAJOR STREAM IN A TURBINE,SUBJECTING BOTH THE MAJOR AND THE MINOR STREAM TO RECTIFICATION IN ARECTIFICATION ZONE AND WITHDRAWING FROM SAID RECTIFICATION ZONE A LOWERBOILING POINT FRACTION AND A HIGHER BOILING POINT FRACTION.